The refrigeration environment is harsh for insulative materials, because of the reactive nature of refrigerant substances, humidity, and temperature extremes. Commonly used insulation for lead wiring in this environment consists of DMD, a wrapped polyester tape disposed between two braids of polyester fibers. This insulation is expensive to manufacture and difficult to apply. The braided fibers of this material tend to cause clogging problems in refrigerant circuits.
Another commonly used insulation material in the refrigeration environment is FEP, which is both expensive and stiff. This insulative substance also tends to present shrinkage/swell problems when exposed to harsher (higher temperature) hermetic requirements.
Still other materials comprise extrudable polyesters and blends of polyesters with up to 30 volume percent of other components, as illustrated in patent Document No. WO 96/34055. The main drawback of these extruded polyesters is their eventual loss of dielectric strength and physical properties, when exposed to refrigerants at elevated temperatures over longer periods of time. Eventually, these materials will crack. Other disadvantages of these materials are their high stiffness and comparative costs. In addition, these insulative substances cannot be crosslinked; they merely melt above their melting temperature. This may be critical for varnish application and baking processes in the manufacture of a refrigerant unit.
In patent Document No. WO 98/05046 an extrusion coating of PBT (polybutyleneterephthalate) with FEP is used to overcome the varnish bake problem.
Fluoropolymers, especially FEP are also in use for motor lead wire insulations. They are both expensive and stiff, and they display swell/shrinkage in certain refrigerants.
Fluoropolymers, especially fluoroelastomers, have been used as gaskets for harsh hermetic environments. These materials, however, are expensive. The fluoropolymers are not crosslinkable, and tend to creep under pressure.
Polyamides have also been used for gaskets, but tend to creep and tend to leach substances, when exposed to the refrigerants.
The new polymer materials of this invention have many advantages over the aforementioned prior art materials. The new compounds show superior property retention on exposure to currently used refrigerants (HCFC-22, HFC-134a). Their dielectric strengths remain unaffected, as do their mechanical characteristics. These materials show good color retention. Swelling and shrinkage of these materials, when used for motor lead wiring, is generally negligible. Swelling of two to five percent has been observed for motor lead wiring with respect to HCFC-123.
Retention of the dielectric constant and physical properties of the materials of this invention are vastly superior to those of prior art materials.
The amount of the leachable fractions of the inventive polymers (which tend to contaminate the refrigerant and eventually clog channels and valves of the refrigeration unit) can be adjusted to acceptable levels by crosslinking. The result is due to the low molecular weight fraction of the polymer, which becomes grafted to the crosslinked network, and due to antioxidants and stabilizers added to achieve sufficient heat age characteristics.
Compared to polyesters and FEP, the inventive compounds have greater flexibility. Due to their excellent retention of mechanical and electrical properties, wires can be produced with reduced insulation thickness, thus reducing cost and weight.
The crosslinked compounds of this invention do not creep under pressure or at elevated temperatures. This is relevant during varnish curing in a motor assembly or at the end application, where temperature peaks cause short circuits.